1. Field
The subject disclosure relates generally to wireless communication and, more particularly, to synchronization channel design for optimal frequency offset estimation.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, data, and so on. These systems may be multiple-access systems capable of supporting simultaneous communication of multiple terminals with one or more base stations. Multiple-access communication relies on sharing available system resources (e.g., bandwidth and transmit power). Examples of multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Communication between a terminal in a wireless system (e.g., a multiple-access system) and a base station is effected through transmissions over a wireless link comprised of a forward link and a reverse link. Such communication link may be established via a single-input-single-output (SISO), multiple-input-single-output (MISO), or a multiple-input-multiple-output (MIMO) system. A MIMO system consists of transmitter(s) and receivers) equipped, respectively, with multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. SISO and MISO systems are particular instances of a MIMO system. A MIMO channel formed by NT transmit and NR receive antennas may be decomposed into Nν independent channels, which are also referred to as spatial channels, where Nν≦min{NT, NR}. Each of the Nν independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput, greater capacity, or improved reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
Regardless the peculiarities of the many available wireless communication systems, in each of these systems a wireless device must perform cell acquisition in order to become operational upon switching on, and tracking to retain communication(s). Cell acquisition is the procedure by which a terminal acquires time and frequency synchronization with the network, cell identification, and additional identification of system information critical to operation, such as system bandwidth and antenna configuration of cell transmitter. It should be appreciated that subsequent to cell acquisition, a mobile terminal can continue to synchronize time and frequency for tracking purposes; e.g., to correct frequency shifts caused by various sources, such as Doppler effect. In sectorized wireless environments, acquisition is to be conducted for each sector present in a cell.
Cell or sector acquisition relies on pilot signals, or acquisition sequences, conveyed through a set of synchronization physical channels and a broadcast channel. Upon transmission of synchronization channels from a cell's or sector's base station, a receiver correlates the acquisition signal with a set of local sequences hypotheses in order to determine time and frequency offsets for receiving downlink traffic. Likewise, a base station can correlate acquisition signals received from a mobile in order to successfully decode control uplink signals. Depending on the utilized acquisition sequences, substantial sensitivity to systematic errors in offset estimation of frequency or time can result in poor downlink or uplink communication. Therefore, there is a need in the art for synchronization channel design that is robust with respect to systematic errors of time offsets and provides optimal frequency offset estimation.